how to select dc motor

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How to select a DC MotorThe different characteristics of each group of DC motors.

By George Hunt MICROMO Clearwater, FL 33762

PMDC Micro-motors

DC motors possess linear relationships that allow for very predictable operation. For

instance, if enough voltage is applied across the terminals of a DC motor, the output shaft

will spin at a rate proportional to that applied voltage. You can take the ratio of the applied

voltage over the rated voltage and multiply that number by the no load speed and get therunning speed. Also, if you decide to measure and plot the current and torque, you will

have a simple straight line indicating yet another directly proportional relationship. When

torque demands increase, so does the current. Plotting the torque and speed together, you

will find that only two points of data are needed. Those are the no load speed and the stall

torque. The entire motion control world, including manufacturers and designers, depend

greatly on the premise that all these linear relationships will hold true. And they do because

the laws of physics do not change! However despite their simplicity, selecting a DC motor

for an application can still be a daunting task. There are many other variables that must be

taken into account including dimensions, load, duty cycle, environment, feedback

considerations, etc. Perhaps decoding some of the mysteries of motor operation will shed

some light on the selection process.

Going for Simplicity with Brushed motors

If your application demands a reliable, time-tested, low cost motor, then brushed DC motor

technology may be what youre looking for. The key here is simplicity. A brushed motor is

designed to run off of straight-line DC voltage and can even be connected directly to a

properly sized battery. When a DC voltage is applied across the terminals of a brushed

motor, a potential difference is achieved and current is induced into the windings on the

rotor. The brushes allow this current to flow through a rotating mechanical switch called a

commutator. The rotor windings act as electromagnets and while powered form 2 poles

that terminate at the commutator segments. This entire assembly is known as an
http://www.dynetic.com/brushless%20terminology.htm#commutationhttp://www.dynetic.com/brushless%20terminology.htm#commutation


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armature. While rotating the commutator allows the direction of the current to reverse two

times per cycle. This permits the current to flow through the armature and the poles of the

electromagnets attracting and repelling the permanent magnets that encompass the motors

inner housing. As the energized windings of the armature pass the permanent magnets,

the polarity of the energized windings reverses at the commutator. This process is called

mechanical commutation and only found in brushed motors. During the instant of switching

polarity, inertia keeps the rotor going in the proper direction and allows the motor to

continue turning. The result is power in its mechanical form measured in watts. Mechanical

power is the product of torque multiplied by the rotational distance per unit time (or speed).

Torque is the force vector component that rotates a load about an axis and is inversely

proportional to speed (see Equation 1).

Eq. 1, where P=Mechanical Power, M=Torque, = angular velocity

From the equation above, we see there is a price to pay for how much power a motor can

deliver. The amount of current that flows through the windings, directly affects to the

torque the motor can produce. Adjusting the supply voltage will force a proportional change

in the motors speed so the output shafts angular velocity (speed) will have to be sacrificed

as torque demands increase. There are also other factors that come into play such as

losses. For example, static friction is defined as the friction torque a motor must overcome

in order for the shaft to begin turning. Then there is brush contact losses caused by the

friction of the brushes upon the commutator. Also, copper losses in the form of heat

sometimes referred to as losses plays a role. Electrical power is represented in Eq. 2.

Eq. 2, where P=Electric Power, I=Current, R=Resistance

Although, when torque and speed are measured empirically, the resulting graph may not be

perfectly linear in all cases. From Eq. 1 however, we can see that both torque and speed

are inversely proportional and that a linear relationship exists. Because of this, feedback

may not even be necessary in all cases. Feedback is usually provided by an encoder,

tachometer or resolver. It tells the servo system where the motor is and what speed the

shaft is turning. Taking all this into account, we can establish that a properly designed

closed loop servo system will have a predictable response to a controlled input. And thanks


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to this directly linear relationship, a servo can easily compensate for any unwanted changes

introduced into the system. See Figure 1 for a plotted curve from a Faulhaber coreless DC

motor. Note specifically the linearity of the torque speed component in Fig. 1.

Iron Core Brushed DC Motors

Traditionally, the motion control industry has relied on iron core brushed DC motors for

demanding applications. They are capable of achieving a very high torque due in part to

their iron core construction. The rotor is usually a rigid design that not only provides a

sturdy support for the windings, but also allows for excellent heat dissipation. That is the

reason more current can be pushed through the windings when torque demands increase.

It acts as a heat sink. Their low cost is yet another plus when project funding is limited.

There are, however some disadvantages to the iron core construction. For example, due to

its heavy armature, overcoming the inertia can reduce the motors acceleration capability.

Higher rotor inertia limits the dynamic characteristics such as the motors acceleration and

stopping time. Another problem with the iron core rotor design is increased inductance.

When running at high speeds the brushes will pass over the commutators segments and

imperfections. At each commutation point, when the brush breaks contact with a

commutator segment, the energy stored in the motor winding as a magnetic field causes anarc or voltage spike between the brush and the commutator segment. This occurs not only

during normal commutation but also in situations where the brushes "bounce" on the

rotating commutator. At higher speeds, this effect can result in faster brush wear and

electro-erosion. One solution is to utilize a precious metal commutator system. This type

of system allows for motors to be manufactured much smaller as a carbon graphite

Figure 1


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commutation system takes up much more space. The commutation signal will usually be

cleaner as well. Since the voltage drop between brushes and commutator is generally small

in precious metal systems, motors can be made to operate at lower voltages. However, due

to a precious metal systems inability to self lubricate, precious metal commutation can

experience a long term effect called micro-welding. This effect can wear down the

commutators surface over time.

Coreless Brushed DC Micro-motors

The answer to some of the problems with iron core technology was addressed in the 1940's

by Dr. Fritz Faulhaber with the invention of the coreless DC micro-motor (see figure 2).

This design opened up a whole new multitude of possibilities for space constrained

applications requiring high precision. These motors have a self-supporting, progressive,

skew-wound, ironless rotor coil that has demonstrated incredible efficiency when compared

to iron core motors. For the first time, DC motors did not require the use of iron

laminations in the armature. Thanks to this construction, the rotor is extremely light

yielding a low moment of inertia. This, in effect, allowed for faster acceleration resulting in

a much smaller mechanical time constant. Another benefit to coreless DC motors is that

they can be manufactured in very compact sizes. That is why they excel in space

constrained applications. The rotor also rotates smoothly without cogging and the coreless

DC motors windings have very low inductance. All of these characteristics help reduce

brush wear and prevent electro-erosion thus increasing the motors lifespan.

Figure 2


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Unfortunately however with no iron laminations coreless motors are somewhat prone to

overheating. In some instances, a heat sink can be used to alleviate this problem. Also,

cost would have to be factored into most applications as the high precision and repeatability

of coreless DC motors comes at a bit of a price. These motors are designed for specific

applications and would not be the best choice to use in most consumer products. The most

common applications are large OEMs in industries requiring very high precision primarily

medical, aerospace, military, robotics and automation. Some example products are

aesthetic lasers, diabetic insulin pumps, collision avoidance scanners, and unmanned aerial

vehicle (UAV) applications. These applications have demanding micro-positioning needs,

dimensional constraints and sometimes vacuum compatibility needs. Coreless DC motors

seem to excel in situations where reliability, precision, longevity and repeatability are of the

upmost importance.

Going for longevity with brushless technology

If an application requires high speed, quiet operation, low EMI and longevity, then brushless

DC technology (BLDC) might be what you are looking for. There are many advantages to

brushless motor technology and speed is one of them. Higher speeds are achievable

because there are no mechanical limitations being imposed by the brushes and commutator.

Another advantage is the elimination of the current arcing/electro-erosion problem

commonly experienced with brushed motors. BLDC motors also possess higher efficiency,

and generate lower EMI which is excellent when used in RF applications. They also possess

superior thermal characteristics over brushed motors since the windings are on the stator.


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The stator is connected to the case, thus the heat dissipation is much more efficient. As a

result, the maintenance on a brushless motor is virtually non-existent.

Unfortunately, the higher cost of construction puts BLDC technology out of reach for many

applications. You can easily spend twice as much on a brushless system and lose the

simplicity of a brushed motor. Dont forget to save room for the control/drive electronics

too. Youll need to mount it somewhere it if it isnt integrated in the motor. Keep in mind,

the motor cant be mounted too far away from the drive as long cable runs tend to

introduce noise into the system. To compensate, the phase leads can be twisted and

shielded from the sensitive feedback leads to reduce noise. As with brushed motors,

brushless must overcome starting friction as well. Again, this is the sum of torque losses

not depending from speed. Dynamic friction is dependent upon speed. In fact, dynamic

torque friction is the only thing defining torque losses proportional to speed for BLDC. A

function of speed (for example in metric units of mNm/rpm), dynamic friction is due to the

viscous friction of the ball bearings, as well as to the eddy currents in the stator originated

by the rotating magnetic field of the magnet.

Overall, you can expect the speed-torque curve to demonstrate excellent linearity for BLDC

technology.

Driving Brushless Micro-motors

Unlike brushed DC motors, brushless technology cannot be operated by connecting directly

up to a straight line DC voltage. Remember, brushless motors utilize electronic

Figure 3


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commutation. So again there are no brushes making physical contact with the commutator.

The permanent magnet rotor initiates motion by chasing a revolving magnetic field induced

by the current in the stator windings. Creating this motion is done with electronics and is

usually an on/off signal called Pulse Width Modulation or PWM. Normally supplied by a

comparator, the PWM signal is a voltage generated as a result of a sinusoidal command

signal and a saw tooth carrier or chopper frequency. The PWM signal is either on or off and

delivered at a duty cycle governed by the chopping frequency. The PWM signal will be high

when the command is greater than the carrier (chopper or switching frequency). The lower

the chopping frequency, the more time the current has to gain amplitude. The motor will

continue to accelerate and decelerate with an accompanying increase in current density.

Such harsh changes in amplitude can result in more ripple in the output as well as

shortened motor life. So it is important that the switching frequency is high enough. The

discrete on/off states are controlled by 6 semiconductor switches which correspondingly

send the amplified current through the correct phase. When the current is reversed by the

semiconductor switches, the stator windings are utilized more efficiently because more than

one winding will be energized. In order to turn the phases on and off at just the right time,

the drive requires feedback. This will help to keep the commutation angle around an ideal

90 degrees. Brushless motors are normally in a closed loop (servo) system to operate

properly. In many cases, digital Hall effects are employed to provide the required feedback

and commutate BLDC motors. For smoother operation, sometimes sinusoidal commutation

(linear Hall effects) can be used.

Micro Positioning with Miniature PMDC Stepper Motors

If precise positioning with the benefits of brushless technology are requirements in an

application, then a permanent magnet DC stepper motor may be just what you need. A

PMDC stepper is a synchronous motor with a magnet rotor and electromagnet stator. The

rotor will usually have 12 pole pairs as will the stator. Normally stepper motors are two

phase, however one, three and even five-phase motors have been developed in the

industry.
http://en.wikipedia.org/wiki/Current_(electricity)http://en.wikipedia.org/wiki/Current_(electricity)


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Steppers are electronically commutated and again brushless, thus they share many of the

same benefits. Relatively immune to the wear and tear of mechanical commutation,

steppers are an excellent choice for positioning applications where response to starting,

stopping and reversing is critical. A very large speed range that can be realized as the

speed is proportional to the input frequency normally supplied by a frequency generator or

drive. PMDC stepper motors possess a small amount of torque even with the coils un-

energized called detent torque. This is due to the magnets interacting with the steel stator.

The rotor will hold its position even without any power being delivered to the motor. In

aerospace applications where power is limited, this attribute has proven useful.

In most instances, feedback is not a necessity.

Open loop operation is quite common in most PMDC stepper applications. Your position can

easily be determined just by keeping track of the input step pulses. This keeps project

costs down and ensures the stepper will have a low profile. It is perfectly acceptable to put

an encoder on a stepper, but not without potential consequences. Stepper motors can

sometimes overshoot the target step and actually oscillate while settling into position. This

is because the rotor is shifting from a de-energized detent state to an energized alignment

state. This phenomenon is due to inertial mismatches and inherent to PMDC steppers. This

resonant behavior can appear to be intermittent if a low resolution encoder is being used.

Figure 4


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Since stepper motors are commutated electronically, drive electronics will have to be

employed. One powerful feature of steppers is their ability to half-step and even micro-

step. Micro-stepping is a driving method where current is continuously varied in the

windings and full steps are divided into many smaller discrete steps. It can be a powerful

feature, but dependent upon whether or not the drive electronics are designed with that

capability. The degree of micro-stepping that can be achieved is governed by the angular

accuracy of the motor. The angular accuracy determines whether you can step, step

and yes even step. This error is non-cumulative and usually around 3 to 5% of a full

step. This specification, like many others, is highly dependent upon the quality of parts and

construction.

Sizing can be a little tricky, but once you understand the way torque is developed in a

stepper it become clear. Note the graph of Fig. 5 where the blue line represents the pull-in

curve. Commonly referred to as the Start/Stop region, the pull-in curve indicates the

maximum frequency a loaded stepper can start and stop instantly without losing

synchronism. The green line defines the area

referred to as the slew rate or pull-out curve. The stepper must be accelerated or

decelerated (ramped) into and out of this region. It cannot be instantaneously started and

stopped because this curve represents the maximum frequency the motor will operate at

Torque-Speed Curve for astepper motor.

Figure 5


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before desynchronizing. The red line represents the mechanical power output. Any inertial

mismatch can change the torque-speed curve significantly. That is why it is recommended

to have a 30 to 50% safety margin when sizing steppers. This can become an issue

especially with applications that require precision. For example, say we connect a battery

driven stepper motor inside a vibrating stuffed animal toy with a load. As the motor

accelerates, the windings will heats up. This causes the temperature coefficient of copper

(Cu) to increase. This in effect raises the winding resistance and changes the way the

stepper will respond. For such a basic application, a slight decrease in performance may be

acceptable. However, in contrast, a pick-and-place robot, an optics control or a

surgical/medical application cannot tolerate unexpected performance variations. For these

situations, another system design may be more suitable. Its recommended that designers

consult manufacturers for assistance in designing these tightly parameterized applications.

Linear motion with actuators

The term linear actuator normally refers to a stepper or

brushless motor with a leadscrew attached to its shaft.

Sometimes a nut and gearbox would be included to form a

compact package designed to deliver precise linear motion (see

figure 6). At the time of development, this was a clever way to

convert rotational to linear motion. Brushless motors work well

when smooth operation coupled with low EMI is desired. Add

high resolution feedback and accurate positioning can be

achieved. Even utilizing a stepper motor can deliver the advantages of instantaneous

starting and stopping. But alas this arrangement is not without its disadvantages. The

conversion from rotary to linear motion is complex one. The problem was that the

mechanical losses in the form of friction were too much to bear for situations where every

bit of power was critical. This is especially true for aerospace applications. This reduced the

efficiency since the linear actuator is not a direct drive mechanism and torque is a vector

component of force.

Figure 6


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This has led to a growing demand in the area of direct drive motion and a need for small

size linear motors. In response to this, new linear servomotors have been developed in the

motion control industry. With the absence of a lead-screw, ball-screw, nut, and friction, this

direct drive unit could apply a purely linear force.

The innovative structure of these motors allows great usage flexibility, tailored to satisfy the

market demand. The self-supporting coil windings together with a high precision sliding

cylinder rod, filled with permanent magnets, provide the motor with a particularly high

performance-to volume ratio. Specially developed calculation software enables easy setting

of the control parameters, displaying specifications, data and graphs of the various profiles.

Position control of the linear DC-servomotor is assured using Motion Controllers MCLM

3003/06 with RS-232 or CAN interface. The Motion Manager software allows quick

configuration of the controllers to optimally run the motor.

Figure 7


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This actuators structure boosts flexibility. It exhibits no residual static force and its output

force is linear with current input, so it is suitable for micro-positioning. If nano-positioning

is required by an application, then a Piezo motor might be the best choice.

How does one prepare for a project that incorporates linear motion? Well, step one is to

define the speed profile for the application at hand. Start by defining the speeds

characteristics of load movements: What is maximum speed? How should mass be

accelerated? What length of movement must the mass traverse? How long is the

applications rest time? If movement parameters are not clearly defined, a triangular or

trapezoidal profile is recommended.

These two values indicate which motors are suitable for the application.

Another option for linear servomotor selection is calculation software, which enables control

parameter setup, specification display, and charts of various profiles. Some motion software

also allows plug-and-play configuration of controllers to optimally run the motor. Many

times if feedback is necessary, then a linear encoder, such as a glass encoder, may be

considered.

For more information, visit micromo.com or call (800) 807-9166.

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how to select dc motor

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